1. Field of Invention
This invention relates to the chemical sensitization of silver halide photothermographic emulsions.
2. Background of the Art
Silver halide-containing photothermographic imaging materials (i.e., heat-developable photographic elements) processed with heat, and without liquid development, have been known in the art for many years. These materials are also known as "dry silver" compositions or emulsions and generally comprise a support having coated thereon: (a) a photosensitive compound that generates silver atoms when irradiated; (b) a relatively or completely non-photosensitive, reducible silver source; (c) a reducing agent (i.e., a developer) for silver ion, for example the silver ion in the non-photosensitive, reducible silver source; and (d) a binder.
Photographic silver halide has its own natural response to radiation, both in wavelength (i.e., spectral sensitivity) and efficiency (i.e., speed). Each of the various pure halides (silver bromide, silver chloride and silver iodide) have their own distinctive wavelengths of sensitivity within the UV, near UV and blue regions of the electromagnetic spectrum. The primary halides used in the formation of photographic silver halides are the chlorides and bromides, with the iodides present as minor proportions, almost always less than 25 molar percent of the total crystal composition. Mixtures of the various silver halides within single grains (e.g., silver chlorobromide, silver chloroiodide, silver bromochloroiodide, silver iodobromide, etc.) would have sensitivities to various different regions of the electromagnetic spectrum, but still within the UV to blue region of the spectrum. The silver halide grains, when constructed and composed of only silver and halogen atoms would also have defined levels of sensitivity based upon their halide content, crystalline morphology (the shape and structure of the crystals or grains), and other artifacts which may or may not have been readily controlled by the silver halide chemist over the years. Such features as crystal defects, crystal stresses, dopants, halide composition, and other structural features have been noted as influential on the sensitometric response of grains and have been purposefully introduced over the years to affect the sensitometry of the emulsions.
The efforts to influence the speed of silver halide grains in general may be broken down into the following categories:
1) Crystal composition, PA1 2) Crystal shape or morphology, PA1 3) Crystal structure, PA1 4) Chemical sensitization (and particularly sulfur sensitization), PA1 5) Reduction sensitization, PA1 6) Dopants, PA1 7) Spectral sensitization, and PA1 8) Supersensitization. PA1 (a) providing a photothermographic emulsion comprising silver halide grains and a non-photosensitive silver source; PA1 (b) providing a sulfur-containing compound positioned on or around the silver halide grains; PA1 (c) sensitizing the silver halide grains by decomposing the sulfur-containing compound on or around the silver halide grains. PA1 (a) preparing a chemically sensitized photothermographic emulsion as described above; PA1 (b) adding a reducing agent and a binder to the photothermographic emulsion; PA1 (c) coating the photothermographic emulsion on a substrate. PA1 (a) providing silver halide grains; PA1 (b) providing a sulfur-containing compound on or around the surface of silver halide grains; PA1 (c) decomposing the sulfur-containing compound thereby chemically sensitizing said grains. PA1 "emulsion layer" means a layer of a photothermographic element that contains the non-photosensitive, reducible silver source and the photosensitive silver halide; PA1 "ultraviolet region of the spectrum" means that region of the spectrum less than or equal to about 400 nm, preferably from about 100 nm to about 400 nm (sometimes marginally inclusive up to 405 or 410 nm, although these ranges are often visible to the naked human eye), preferably from about 100 nm to about 400 nm. More preferably, the ultraviolet region of the spectrum is the region between about 190 nm and about 400 nm; PA1 "short wavelength visible region of the spectrum" means that region of the spectrum from about 400 nm to about 450 nm; PA1 "infrared region of the spectrum" means from about 750 nm to about 1400 nm; preferably from about 750 nm to about 1000 nm. PA1 "visible region of the spectrum" means from about 400 nm to about 750 nm; and PA1 "red region of the spectrum" means from about 600 nm to about 750 nm. Preferably the red region of the spectrum is from about 630 nm to about 700 nm.
The first three mechanisms have been briefly described above.
Chemical sensitization is a process during the crystal making process in which sensitizing specks of materials such as silver salts (e.g., Ag.sub.2 S) or even silver metal are introduced onto (usually) or into the individual grains. The introduction of silver sulfide specs, for example, is usually done by direct reaction of active sulfur contributing compounds with the silver halide during various stages in the silver halide growth process. The presence of the specks increases the speed or sensitivity of the grains to light and/or development. The first observation of sulfur sensitization came from early findings that different gelatin binders would often produce different degrees of sensitivity in silver halide emulsions, so the source of the speed increasing component was investigated and found to be sulfur contributing compounds. Thiosulfate compounds are still typically used as a labile sulfur compound. Other materials such as allylthiourea are also used. Certain studies (e.g., by Sheppard, Trevelli and Wightman J. Franklin Inst., 1923, 196, 653,673) using micrography, found that the treatment of silver halide grains with allylthiourea solution followed by carbonate solution resulted in the formation of black specks rather than a distribution of silver halide over the grain surface (Mees and James, The Theory of the Photographic Process, 4th edition, 1977, p. 152.). It has also been suggested that the thiourea rearranges itself on the surface of the grains to active configurations in the generation of silver sulfide specks (Mees and James, supra, p. 153). It has also been suggested that the thiosulfate acts to sensitize the silver halide by AgSO.sub.3.sup.- adsorbed to the crystal surface.
Reduction sensitization is somewhat similar to chemical sensitization, but distinguishable therefrom, and is a process by which other chemical species, besides silver sulfide, are deposited or reacted into or onto the silver halide grains during a segment of the silver halide grain growth and finishing steps. The term reduction sensitization, although generically considered within the term of chemical sensitization, refers specifically to describe emulsions sensitized by the action of reducing agents on the silver halide grains. Materials which have been used as reduction sensitizers include stannous chloride, hydrazine, ethanolamine, and thioureaoxide.
Dopants most importantly include gold sensitization where the silver halide grains are treated with gold containing ions such as tetrachloroaurate (III) or dithiocyanurate(I). Thiocyanate has been suggested as being capable of increasing gold sensitization (Mees and James, supra, p.155). The gold is most preferably added at the later stages of silver halide grain formation, such as during ripening, after grain growth. Other metals such as platinum and palladium are also known in the art to have some effects similar, but not as specifically beneficial as gold. Still other metal dopants such as iridium, rhodium, ruthenium and the like are known more for contrast or high intensity reciprocity effects than for speed sensitization effects.
Spectral sensitization is the addition of compounds to silver halide grains which absorb radiation at wavelengths other than those to which silver halide is naturally sensitive (i.e., only within the UV to blue) or which absorb radiation more efficiently than silver halide (even within those natural regions of spectral sensitivity). It is generally recognized that spectral sensitizers extend the responses of photosensitive silver halide to longer wavelengths and can accomplish spectral sensitization in the UV, visible or infrared regions of the electromagnetic spectrum. These compounds, after absorption of the radiation, transfer energy to the silver halide grains to cause the necessary local photoinduced reduction of silver salt to silver metal. The compounds are usually dyes, and the best method of spectrally sensitizing silver halide grains causes or allows the dyes to align themselves on the surface of the silver halide grain, particularly in a stacked, almost crystalline pattern on the surface of the individual grains.
Supersensitization is a process whereby the speed of a spectrally sensitized photographic silver halide is increased by the addition of another compound, which may or may not be a dye. This is not merely an additive effect of two compounds, as it is understood in the art. For example, where two separate dyes are used, one as the spectral sensitizer and the other as a supersensitizer, the surface of the grain still may not have more than a defined amount of dye present, yet the combination of the two dyes will provide a speed which is superior to that of either dye alone, even when optimized.
These various speed enhancing processes may of course be combined in the formulation of a specific photographic emulsion, as the situation requires.
In photothermographic emulsions, the photosensitive compound is generally photographic silver halide which must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver atoms (also known as silver specks, clusters, or nuclei) are generated by irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source within a catalytic sphere of influence around the silver specs. It has long been understood that silver atoms (Ag.degree.) are a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions. The silver halide may be made "in situ," for example by adding a halogen-containing source to the reducible silver source to achieve partial metathesis (see, for example, U.S. Pat. No. 3,457,075); or by coprecipitation of silver halide and the reducible silver source (see, for example, U.S. Pat. No. 3,839,049). The silver halide may also be pre-formed (i.e., made "ex situ") and added to the organic silver salt. The addition of silver halide grains to photothermographic materials is described in Research Disclosure, June 1978, Item No. 17029. The reducible silver source may also be generated in the presence of these ex situ, pre-formed silver halide grains. It is reported in the art that when silver halide is made ex situ, one has the possibility of controlling the composition and size of the grains much more precisely, so that one can impart more specific properties to the photothermographic element and can do so much more consistently than with the in situ technique.
The non-photosensitive, reducible silver source is a compound that contains silver ions. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic compounds, such as silver imidazolates, have been proposed. U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image is generally not visible by ordinary means. Thus, the photosensitive emulsion must be further processed to produce a visible image. This is accomplished by the reduction of silver ions which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms, (i.e., the latent image). This produces a black and white image. In photographic elements, the silver halide is reduced to form the black-and-white negative image in a conventional black-and-white negative imaging process. In photothermographic elements, the light-insensitive silver source is reduced to form the visible black-and-white negative image while much of the silver halide remains as silver halide and is not reduced.
The reducing agent for silver ion of the light-insensitive silver salt, often referred to as a "developer," may be any compound, preferably any organic compound, that can reduce silver ion to metallic silver, and is preferably of relatively low activity until it is heated to a temperature above 100.degree. C. At elevated temperatures, in the presence of the latent image, the non-photosensitive reducible silver source (e.g., silver behenate) is reduced by the reducing agent for silver ion. This produces a negative black-and-white image of elemental silver.
While conventional photographic developers such as methyl gallate, hydroquinone, substituted-hydroquinones, catechol, pyrogallol, ascorbic acid, and ascorbic acid derivatives are useful, they tend to result in very reactive photothermographic formulations and fog during preparation and coating of photothermographic elements. As a result, hindered phenol developers (i.e., reducing agents) have traditionally been preferred.
As the visible image in black-and-white photothermographic elements is usually produced entirely by elemental silver (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable to reduce the cost of raw materials used in the emulsion and/or to enhance performance. For example, toning agents may be incorporated to improve the color of the silver image of the photothermographic elements as described in U.S. Pat. Nos. 3,846,136; 3,994,732; and 4,021,249.
Another method of increasing the maximum image density in photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming or dye-releasing compounds in the emulsion. Upon imaging, the dye-forming or dye-releasing compound is oxidized, and a dye and a reduced silver image are simultaneously formed in the exposed region. In this way, a dye-enhanced black-and-white silver image can be produced. Dye enhanced black-and-white silver image forming elements and processes are described in, for example, U.S. Pat. No. 5,185,231.
Many cyanine and related dyes are well known for their ability to impart spectral sensitivity to a gelatino silver halide element. The wavelength of peak sensitivity is a function of the dye's wavelength of peak light absorbance. While many such dyes provide some spectral sensitization in photothermographic formulations, the dye sensitization is often very inefficient and it is not possible to translate the performance of a dye in gelatino silver halide elements to photothermographic elements. The emulsion making procedures and chemical environment of photothermographic elements are very harsh compared to those of gelatino silver halide elements. The presence of large surface areas of fatty acids and fatty acid salts restricts the surface deposition of sensitizing dyes onto silver halide surfaces and may remove sensitizing dye from the surface of the silver halide grains. The large variations in pressure, temperature, pH and solvency encountered in the preparation of photothermographic formulation aggravate the problem. Thus sensitizing dyes which perform well in gelatino silver halide elements are often inefficient in photothermographic formulations. In general, it has been found that merocyanine dyes are superior to cyanine dyes in photothermographic formulations as disclosed, for example, in British Patent No 1,325,312 and U.S. Pat. No. 3,719,495. Recently, certain cyanine dyes have been disclosed as spectral sensitizers for use in photothermographic elements. For example, U.S. Pat. Nos. 5,441,866 and 5,541,054 describe photothermographic elements spectrally sensitized with benzothiazole heptamethine dyes substituted with various groups, including alkoxy and thioalkyl.
Although spectral sensitizing dyes for photothermographic elements are now known which absorb throughout the visible and near-infrared regions (i.e., 400-850 nm) photothermographic emulsions which provide higher photospeeds and which have improved shelf-life stability, sensitivity, contrast and low Dmin are still needed for photothermography.
U.S. Pat. No. 4,207,108 (Hiller) describes improved speed in photothermographic materials by addition of a photographic speed increasing concentration of a certain non-dye, thione speed increasing addendum (including compounds with cyclic thiocarbonyl &gt;C.dbd.S! groups within the cyclic structure). No decomposition of the cyclic thione compounds is reported.
U.S. Pat. No. 5,541,055 (Ooi et al.) describes photothermographic elements which comprise both a cyanine dye and a colorless cyclic carbonyl compound. Rhodanine, hydantoin, barbituric acid, or derivatives thereof (all shown to be monocyclic in columns 4-6) are particularly preferred as the colorless cyclic carbonyl compound.
The recent commercial availability of relatively high powered semiconductor light sources, and particularly laser diodes which emit in the red and near-infrared region of the electromagnetic spectrum, as sources for output of electronically stored image data onto photosensitive film or paper is becoming increasingly widespread. This has led to a need for high quality imaging articles which are sensitive at these wavelengths and has created a need for more highly sensitive photothermographic elements to match such exposure sources both in wavelength and intensity. Such articles find particular utility in laser scanners.
Differences Between Photothermography and Photography
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic elements differ significantly from conventional silver halide photographic elements which require wet-processing.
In photothermographic imaging elements, a visible image is created by heat as a result of the reaction of a developer incorporated within the element. Heat is essential for development and temperatures of over 100.degree. C. are routinely required. In contrast, conventional wet-processed photographic imaging elements require processing in aqueous processing baths to provide a visible image (e.g., developing and fixing baths) and development is usually performed at a more moderate temperature (e.g., 30-50.degree. C.).
In photothermographic elements only a small amount of silver halide is used to capture light and a different form of silver (e.g., silver behenate) is used to generate the image with heat. Thus, the silver halide serves as a catalyst for the physical development of the non-photosensitive, reducible silver source. In contrast, conventional wet-processed black-and-white photographic elements use only one form of silver (e.g., silver halide); which, upon chemical development, is itself converted to the silver image; or which upon physical development requires addition of an external silver source. Additionally, photothermographic elements require an amount of silver halide per unit area that is as little as one-hundredth of that used in conventional wet-processed silver halide.
Photothermographic systems employ a light-insensitive silver salt, such as silver behenate, which participates with the developer in developing the latent image. In contrast, chemically developed photographic systems do not employ a light-insensitive silver salt directly in the image-forming process. As a result, the image in photothermographic elements is produced primarily by reduction of the light-insensitive silver source (silver behenate) while the image in photographic black-and-white elements is produced primarily by the silver halide.
In photothermographic elements, all of the "chemistry" of the system is incorporated within the element itself. For example, photothermographic elements incorporate a developer (i.e., a reducing agent for the non-photosensitive reducible source of silver) within the element while conventional photographic elements do not. The incorporation of the developer into photothermographic elements can lead to increased formation of "fog" upon coating of photothermographic emulsions. Even in so-called instant photography, the developer chemistry is physically separated from the photosensitive silver halide until development is desired. Much effort has gone into the preparation and manufacture of photothermographic elements to minimize formation of fog upon coating, storage, and post-processing aging.
Similarly, in photothermographic elements, the unexposed silver halide inherently remains after development and the element must be stabilized against further development. In contrast, the silver halide is removed from photographic elements after development to prevent further imaging (i.e., the fixing step).
In photothermographic elements the binder is capable of wide variation and a number of binders are useful in preparing these elements. In contrast, photographic elements are limited almost exclusively to hydrophilic colloidal binders such as gelatin.
Because photothermographic elements require thermal processing, they pose different considerations and present distinctly different problems in manufacture and use. In addition, the effects of additives (e.g., stabilizers, antifoggants, speed enhancers, sensitizers, supersensitizers, etc.) which are intended to have a direct effect upon the imaging process can vary depending upon whether they have been incorporated in a photothermographic element or incorporated in a photographic element.
Because of these and other differences, additives which have one effect in conventional silver halide photography may behave quite differently in photothermographic elements where the underlying chemistry is so much more complex. For example, it is not uncommon for an antifoggant for a silver halide system to produce various types of fog when incorporated into photothermographic elements.
Distinctions between photothermographic and photographic elements are described in Imaging Processes and Materials (Neblette's Eighth Edition); J. Sturge et al. Ed; Van Nostrand Reinhold: New York, 1989, Chapter 9; in Unconventional Imaging Processes; E. Brinckman et al, Ed; The Focal Press: London and New York: 1978, pp. 74-75; and in C. Zou, M. R. V. Shayun, B. Levy, and N. Serpone J. Imaging Sci. Technol. 1996, 40, 94-103.